SWI-Prolog -- tspy/1

Documentation
- Reference manual
  - Multithreaded applications
    - Thread support library(threadutil)
      - Debugging threads
        
        attach_console/0
        
        tdebug/1
        
        tdebug/0
        
        tnodebug/1
        
        tnodebug/0
        
        tspy/2
        
        tspy/1
- Packages

Availability::- use_module(library(threadutil)).(can be autoloaded)

Source

tspy(:Spec)

Set a spy point as spy/1 and enable debugging in all threads using tdebug/0. Note that removing spy points can be done using nospy/1. Disabling spy points in a specific thread is achieved by tnodebug/1.